One of engineering's oldest problems is how to make a structure of the lightest materials and containing the least number of parts but retaining the necessary strength. It is well known that a structure may be made to carry a greater load if it is prestressed to oppose the working stresses. There are many ways to eliminate excess material in a structure. In the subject invention this has been accomplished by changing the geometry to arrange the parts of the structure in a more effective way whereby a lesser number of parts each work to their allowable limit thereby eliminating excess parts and the consequent additional weight and complexity. This has been accomplished by the subject invention by prestressing the fabric or cable restraints without consequent twisting or buckling of the panel.
In developing the subject device, a structure was formed by attaching a skin or fabric member to opposite ends of rod members which rod members were forced apart by two resilient poles to place a force on the rod members to thereby prestress the skin. It was found that the device, when placed over an excavation, such as a foxhole, was able to support more than twenty inches of dirt completely covering the skin without permanent deformation of any of the components of the structure. The result was that the resilient poles were further compressed and the rod members were drawn toward each other and the skin assumed a catenary shape and extended into the excavation. This fault has been eliminated by employing a single, slightly pre-bowed stress-producing member made of spring material over which the skin member is disposed. When this device was loaded, both by dirt or by a wind load, in the case of an enclosure, the skin acted against the pre-bowed stress-producing member to restrain further outward bowing thereby eliminating the catenary problem.
In accordance with this invention, controlled longitudinal extension is provided in the panel structure by means of a chord compression system. In its simplest form, namely a rectangular structure, a pair of spaced-apart, slightly bowed rod members are provided. A flexible cable restraint, or fabric restraint, or both is secured to at least each of the terminal ends of the rod members. A stress-producing member is interposed between the rod members forming thereby a prestressed framework. When a skin or fabric restraint member is secured to the rod members and or cable members and disposed on top of the bowed stress-producing member and an external dynamic load is applied, such as by the wind in the case of a shelter, or by a static load, such as dirt in the case of a foxhole cover, the forces generated by the loading are opposed by the stress-producing member and equilibrium is reached without any considerable alteration in the original external shape or dimensions of the panel.
It will be seen from the above that the panel structure of this invention consists, essentially, of three members, which are, a stress-producing member, a restraint member and a means to transmit a uniform loading from the stress member to the restraint member.